There was a time when I created some letter series puzzles. This one is an old one. Replace the question marks.

P, Ca, ?, ?, O, Ca, P, Ca, ?, ?, ?, Ca

Hints(1-3): I will reveal three question marks. Not in order.

Hint 1:


Hint 2:


Hint 3:


Hint 4:


Hint 5:

But there was no bicycle at that time

Hint 6:

In the meanwhile my puzzles became more diverse

  • 3
    $\begingroup$ I see a number of chemical elements. $\endgroup$
    – Nick
    Nov 18, 2019 at 10:15
  • $\begingroup$ Can you give a small hint? $\endgroup$ Nov 19, 2019 at 6:57
  • $\begingroup$ @archipelago, Could you please use the "@" sign before an username in comments? $\endgroup$
    – Nick
    Nov 19, 2019 at 11:13
  • $\begingroup$ Before starting real hints here are some "sub-hints": The most @Nick could see is symbols (of chemical elements.) No prime numbers will be involved in this puzzle. After somebody removed the lateral thinking tag, I put it back. $\endgroup$
    – user63710
    Nov 19, 2019 at 11:19
  • $\begingroup$ P, Ca, O, P, O, Ca, P, Ca, O, O, P, Ca $\endgroup$
    – Nick
    Nov 19, 2019 at 15:10

2 Answers 2


This may be a stretch as this seems mostly irrespective of the ordering of the sequence.

P, Ca, C, Si, O, Ca, P, Ca, Si, O, C, Ca


The first five elements are the constituent parts of the chemical reaction of Calcium Phosphate, Carbon Coke, and Silicon Dioxide: P, Ca, C, Si, O. The latter seven elements give the products (and this is where it definitely disregards the ordering in the reaction): Ca, P, Ca, Si, O, C, Ca. I think the three occurrences of Ca in the second subsequence is indicative of the yield including three Calcium Silicates. Link to a page with the reaction: https://chemiday.com/en/reaction/3-1-0-794

  • $\begingroup$ no, but upvoted for giving insight to some chemistry $\endgroup$
    – user63710
    Nov 22, 2019 at 16:10

Here’s my answer:

P, Ca, O, Si, O, Ca, P, Ca, O, Si, 0, Ca

This is the pattern as it can be formed as you said you have revealed three answers to replace the question mark.

  • 3
    $\begingroup$ Technically this is just one of the possible patterns that can be formed by adding the three hinted symbols (since there are still two unknowns, and since these unknowns could be paired in the same position within the two halves, they could be any pair of matching symbols - if indeed both halves should match at all). So why this particular pattern, please? Thanks :) $\endgroup$
    – Stiv
    Dec 7, 2019 at 7:06

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